Wireless communication networks typically make a number of ongoing service decisions based on the channel qualities associated with the individual users being served. For example, users experiencing better channel conditions may be served in preference to users experiencing poor channel conditions mil/or may be served at higher data rates. In general, newer wireless communication networks may schedule uplink and/or downlink transmission resources in dependence on the channel qualities reported by the current crop of active users.
As wireless communication networks evolve, the range and sophistication of transmit link adaptations made responsive to channel quality increases. For example, in wireless communication networks that use Multiple-Input-Multiple-Output (MIMO) transmission techniques, the decision to use MIMO and the particular MIMO configuration adopted for transmissions to or from a given user may be based at least in part on the channel conditions experienced by that user.
In Long Term Evolution (LTE) wireless communication, there are two channels normally used for sending channel quality information (also referred to as channel quality feedback). One such channel is the Physical Uplink Control Channel (PUCCH) and the other such channel is the Physical Uplink Shared Channel (PUSCH). The PUCCH generally is used for sending periodic channel quality reports, while the PUSCH may be used for sending channel quality information in conjunction with uplink data transmissions.
One or both such channels support different modes of channel quality reporting, also referred to as Channel State Information (CSI) reporting. As a general proposition, a individual user may report a wideband Channel Quality Indicator (CQI) that indicates to the serving LTE base station (an eNodeB) the overall channel quality across the system bandwidth. Here, the user may be essentially any type of access equipment, such as cellular phone, wireless modem, etc. Further, the “system” bandwidth may, in one or more examples, be understood as the aggregate bandwidth of the overall Orthogonal Frequency Division Multiplexing (OFDM) carrier that is used for serving users in the downlink.
In one or more related modes, the user reports a wideband CQI but additionally reports frequency-selective Precoder Matrix Indicators (PMIs). For example, the system bandwidth may be subdivided into multiple subbands, with a PMI value reported for each subband. These PMIs will be understood as the user's recommendations for precoding multi-antenna transmissions to the user within each such subband. The serving base station may or may not follow those recommendations but they nonetheless represent the individual user's frequency-selective recommendations based on, for example, the user evaluating channel conditions within each defined subband of the overall system bandwidth.
Conversely, rather than reporting a wideband CQI with or without frequency-selective PMIs, an individual user may report frequency-selective CQIs, with each reported CQI representing the channel quality estimate for a given one of the defined frequency subbands. The frequency-selective CQIs may be reported with a wideband PMI, representing a precoder selection recommendation that is generally applicable to the whole system bandwidth.
Thus, the various channel quality reporting modes may be understood as striking different balances between the granularity of PMI reporting versus CQI reporting. Two CSI modes have been shown to perform well for MIMO in previous LTE network studies, e.g. PUSCH 1-2 and PUSCH 3-1. PUSCH 3-1 includes frequency selective CQIs, thus enabling frequency selective scheduling. The “−1” denotes that the report includes only one PMI for the whole bandwidth. PUSCH 1-2 has only a single wideband CQI, and instead reports multiple PMIs. For example details CSI reporting modes in LTE, refer to ETSI TS 136 213 V10.1.0 (2011-04), LTE; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures (referred to as 3GPP TS 36.213 version 10.1.0 Release 10).
CQI mode control may be carried out using Radio Resource Control (RRC) protocol signaling. RRC signaling is performed over the data channel and, as it is control information represents overhead, it should be kept to a minimum. On the other hand, the system should be able to quickly reconfigure the CSI reporting mode, e.g., on the order of tens of milliseconds. For example details regarding CSI mode configuration, again refer to TS 36.213, with particular reference to Section 7.2.